1. Field of the Invention
This invention relates to a powder mixture for monolithic refractories containing graphite and a method of making thereof utilized in a lining of a vessel which is used in the processes of iron making and steel making, and its repair.
2. Discussion of the Invention
Shaped refractories containing graphite are widely used in the field of iron making and steel making since they have preferable properties such as refractoriness and thermal shock resistance, and are hardly wettable to molten iron, molten steel and molten slag, showing an excellent corrosion resistance. However, the conventional shaped refractories containing graphite which are preformed and baked or fired have an excellent performance, many steps of operation and much energy are required in the production and the operation thereof. Accordingly, much cost and operational time are required as a whole. On the other hand, labor-saving is a necessary condition in the business of refractories, for maintaining an enterprising activity. Therefore, the use of monolithic refractories is increasing as a general tendency.
The monolithic refractories are operated by adding a dispersion media such as water to a premixed powder mixture in the operational field. They are operated by methods of gunning, casting with vibration, stamping and the like. As the dispersion media, it is possible to utilize an organic solvent having a good wettability with respect to graphite particles. However, in view of the working environment and the cost thereof, it is the best method to utilize water as the dispersion media.
When water is used as the dispersion media for monolithic refractories containing graphite, since the graphite particles are devoid of a hydrophilic property (the same with the dispersive property with respect to water), a molded body having a large bulk density can not be obtained as in the case of other many monolithic refractories. Accordingly, the obtained monolithic refractories containing graphite are considerably inferior to shaped refractories in properties such as oxidation resistance, corrosion resistance or strength, and refractories having a long service life cannot be obtained, which is a hazard against the practical utilization thereof.
Only little functional groups having a hydrophilic property such as --OH, --COOH or the like are existent on surfaces of the graphite particles. The surface has a hexagonal mesh structure of carbon atoms having no hydrophilic property. The absolute value of .zeta.-potential on the interface thereof with water is small when water is utilized as the dispersion media. Therefore, the graphite particles have a poor hydrophilic property. As methods to avoid the problems of the graphite particles having a poor hydrophilic property, that is, inferior dispersive property with respect to water, strong acid treatment method, CVD (chemical vapor deposition) method, sol-gel method, polymer coating method and the like have been investigated. However, no satisfactory result has been obtained by any one of these methods.
In the strong acid treatment method, a graphite powder is immersed in concentrated sulfuric acid, concentrated nitric acid, hydrofluoric acid or the like, and reacted with these acids at room temperature to 100.degree. C., thereby making the surfaces of the graphite particles hydrophilic. However, the acid components penetrate in the graphite crystal and intercalation compounds are formed, whereby the graphite particles are expanded and the acid components remaining in the graphite crystals dissolve into water added as the dispersion media, thereby changing the pH value and deteriorating the dispersed state or making it unstable. Accordingly, when the graphite-base powder (hereinafter a surface improved graphite powder is called a graphite-base powder) is utilized as a raw material powder of monolithic refractories, a formed body having a large bulk density can not be obtained.
In the CVD method, a gas component such as SiO or B.sub.2 O.sub.3 is brought in contact with the surfaces of the graphite particles for instance, at 1000.degree. C. or more, thereby forming thin films of SiC or B.sub.4 C on their surfaces. By this reaction, the surfaces of the graphite particles are oxidized and become a porous rugged structure thereby lowering the oxidation resistance of the graphite particles. As a substitute method, halide CVD method is well known. However, since the raw material gas thereof is expensive and an emission gas treatment is necessary, the method is not suitable for treating raw material powders of refractories having a low added value.
In the sol-gel method, for instance, silicon alkoxide, aluminum alkoxide or the like is hydrolyzed in an alcoholic aqueous solution in the presence of an acidic catalyst. The obtained sol solution is impregnated in a graphite powder, which is dried to be a gel, thereby attaching SiO.sub.2 or Al.sub.2 O.sub.3 gel film to the surfaces of the graphite particles. However, since the wettability of the graphite particles with respect to the sol solution is poor, vacancies are formed between the graphite particles and gel films, and much surface portions which are not covered by the gel films, remain. Accordingly, a sufficient dispersive property of the graphite base powder with respect to water is not obtained and therefore, a molded body having a large bulk density can not be obtained when they are formed using water as the dispersion media.
In the polymer coating method, disclosed for instance in Japanese Examined Patent Publication No. 33666/1990, solution of phenol resin, furanol resin, silicone resin or the like is impregnated in a graphite powder thereby forming coatings of organic resin on the particle surfaces of the graphite powder. As in the case of solgel method, the resin coated graphite-base powder is devoid of the water wettability and become porous since the resin is thermally decomposed in their use. Accordingly, a formed body of a monolithic refractory containing graphite having a good service life can not be obtained.
As a conventional trial for obtaining a graphite powder having hydrophilic property used for the monolithic refractories containing graphite, there is a description for instance, in Japanese Examined Patent Publication No. 46473/1989. By making a pelleted graphite-base powder wherein a thermosetting resin or a thermoplastic resin is mixed with a graphite powder and the mixture is pressed, or the pressed body is broken, the dispersion property with respect to water is improved.
The hydrophilic property of the graphite particles can be improved to some degree in any one of the above methods. However, most of the organic resins coated on the graphite particles or taken in the graphite particles, are thermally decomposed in their use and discharged as gases, thereby increasing the porosity of refractories and deteriorating the oxidation resistance and the corrosion resistance which are important properties in these kinds of refractories.
Furthermore, in the above proposals, there is a description wherein particle surfaces of aluminum or silicon are covered simultaneously with surfaces of graphite particles by an organic resin. In this case, the hydrophilic property of the organic resin which is not necessarily good regulates the hydrophilic property of these powders.
As a recent technology, a surface improving method of solid particles is proposed in Japanese Examined Patent Publication No. 2009/1991. As a preferable method of attaching particles having a particle size in a range of 0.01 to 10 .mu.m to surfaces of other particles having a particle size in a range of 0.1 to 100 .mu.m, a method is proposed wherein a mechanical impact force is utilized. As a substantial means, a method of mechanical impact treatment in a high-speed gas flow, is disclosed. However with respect to particles of a graphite powder having a particular cleavage, no suggestion is presented in the specification to whether the surface improvement of the natural flake graphite particles is possible by this method.